Two essential components of a digital scanner, utilized for example in color image reading systems, are a lamp and a charge coupled device (CCD). In spite of their importance in system performance, frequently these elements are chosen on the basis of their individual characteristics, with little regard to how the two will function together.
For example, the CCD may be chosen with a view to its sensitivity and high signal to noise (S/N) ratio while brightness and stability are criteria usually considered when the lamp is chosen. The result can be a mismatch between the output spectrum of the lamp and the CCD sensitivity so that the signal strength of the red, green and blue (RGB) channels of the CCD are unbalanced.
In this regard, the CCD will saturate if driven above a maximum voltage. This places a limit upon exposure, which is dictated by the high output RGB channel. In such a case, the low output channel is sampled at a lower output voltage and, consequently, at a lower signal to noise ratio.
The problem in providing a combined illumination that compensates for the spectral luminous efficiency of conventional image processing devices has been recognized in the prior art and various solutions have been advanced. For example, in U.S. Pat. No. 4,713,683, a system for illuminating and synchronizing color imaging equipment is disclosed. In the system, the illuminating light is irradiated from a light source whose current can be changed. Three color filters are utilized and the light source current can be arbitrarily changed so that the intensity of light of each of the three colors can be controlled to optimum values, through filter selection and/or electrical current adjustment. The system described in the aforesaid patent is complicated, requiring a timing mechanism and a synchronization signal generator to control filter disposition.
In U.S. Pat. No. 4,679,073, there is disclosed a color image reading apparatus for controlling the intensity of a light source illuminating a color document. While this apparatus has some utility, it has limitations also since three separate scans are required, with appropriate lamp brightness adjustments being made for each scan.
Finally, it is known in the prior art that alteration of phosphors in a fluorescent lamp is capable of producing a light of predetermined ICI coordinates. For example, in U.S. Pat. No. 4,176,299, a method is set forth wherein lamp phosphors are adjusted in order to produce a light, perceived by the human eye as approximating that of daylight.
However, the problem is not so easily solved in color imaging systems. A lamp spectrum providing daylight-equivalent illumination for the human eye will not balance RGB output from a color CCD. The greater the imbalance among the three channels, the lower the S/N for the low output channel, since system brightness is limited by the high-output channel.
In view of the foregoing, it would be advantageous to have a color imaging system, and a process for constructing such a system, wherein the spectral output of the lamp is tuned to enable all three CCD channels to operate at the high end of their respective output ranges, thereby maximizing signal to noise ratios. Preferably, such a system, and process, would make it possible to avoid the limitations of conventional, unbalanced systems in which the brightest channel limits total system brightness. Ideally, the process would enable such balancing that the three primary colors in a color document could be scanned simultaneously, without degradation of system performance.